The present invention relates to a moisture-reactive composition which can form a transparent and flexible moisture absorbing layer, and an organic electroluminescence (hereinafter abbreviated to EL) device including drying means formed of the composition.
An organic EL device utilizing an organic material has attracted special interest as a light emitting device capable of emitting light with high luminance due to DC drive at low voltage, the light emitting device comprising an anode, a cathode and an organic layer formed between the anode and the cathode, the organic layer comprising an organic charge transporting layer and an organic light emitting layer laminated with each other. Also this organic EL device is expected as a flexible display because the entire material can be composed of a solid.
On the other hand, the organic EL device has such a problem that when the organic EL device is driven for a fixed period, light emitting characteristics such as light emitting luminance, luminous efficiency and luminous uniformity drastically deteriorate as compared with the case of an initial stage. Examples of the cause of deterioration of these light emitting characteristics include oxidation of electrodes caused by oxygen penetrated into the organic EL device, oxidative decomposition of an organic material caused by heat generation during driving, and denaturation of an organic matter. The cause of deterioration of light emitting characteristics further includes mechanical deterioration of a structure, for example, peeling of the interface of the structure is caused by an influence of oxygen and moisture and also heat generation during driving and high-temperature environment trigger generation of stress at the interface of the structure due to a difference in a thermal expansion coefficient of the respective components, resulting in peeling of the interface.
To prevent these problems, there have been studied various techniques of sealing an organic EL device in order to inhibit contact with moisture and oxygen. For example, as shown in FIG. 1, there is disclosed a method of preventing arrival of moisture at an organic EL device by putting a sealing cap 2 with a water absorbing agent 6 adhered on the inner wall on a picture element area including a substrate 1 and the organic EL device comprising a transparent electrode 3, an organic functional layer 4 and a metal cathode electrode 5 formed on the substrate, filling the inside with a nitrogen gas 9, and fixing the sealing cap on the substrate 1 using an adhesive 7 (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-148066). Also there is disclosed a method of reducing an influence of oxygen by using an oxygen absorbing agent in place of the water absorbing agent (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 7-169567).
Various substances have been studied as water absorbing agents. Among various substances, alkali earth metal oxides such as barium oxide (BaO) or calcium oxide (CaO) have widely been studied because they can certainly capture water molecules by the chemical reaction and do not release water molecules at high temperature, unlike water absorbing agents such as silica gel or zeolite which physically adsorb water.
However, these water absorbing agents have drawbacks because they are particles of an inorganic compound and require a concave substrate so as to adhere in the device and therefore the resulting device is thick. Also, alkali earth metal oxides are opaque and therefore can be applied to a so-called bottom emission type display device in which display light is taken out from the side of the substrate 1. However, when the alkali earth metal oxide is applied to a so-called top emission type display device in which display light is to be emitted from the side of the sealing cap 2 opposite the substrate 1, emission of light is obstructed by the water absorbing agent 4, and therefore the water absorbing agent 4 must be arranged so as not to enter in the image picture area and an installation location must be provided.
Some proposals have been made for applying water absorbing agents to top emission type display devices. For example, it can be easily considered to apply polymers such as polyvinyl alcohol and nylon, which are transparent and have water absorbing properties, as the water absorbing agent. However, these polymers physically adsorb water and do not have sufficient water absorbing properties as described above. It is also proposed to use a particulate water absorbing agent arranged so as not to adversely affect light transmission properties in a top emission type structure (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2001-357973). It is also proposed to use a plastic substrate in which a water absorbing agent having a particle size smaller than an emission wavelength of the organic EL device is dispersed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-56970). However, inorganic particles are difficult to arrange and uniformly disperse as primary particles, and thus deterioration of transmission caused by light scattering cannot be avoided.
As means for solving these problems, the use of a water capturing film which absorbs less visible light, but has sufficient transparency, has been proposed (see Japanese Unexamined Patent Publication (Kokai) No. 2003-142256). This water capturing film can be formed by coating a special metal compound dissolved in a solvent. However, when applied to a flexible substrate, the water capturing film is insufficient in flexibility because the water capturing film is made of a low molecular weight compound. After moisture absorption, the compound becomes more brittle and may adhere on the organic EL device as a dust.